We have investigated sialidase activities in transformed rat 3Y1 cells of different metastatic potential. Only lysosome-type sialidase was apparent in the particulate fractions of 3Y1 cells and their transformants. As compared with control 3Y1 cells, src-transformed cells exhibited decreased sialidase activity, and v-fos transfer to these latter induced even more severe decrease in the sialidase activity with acquisition of high lung metastatic ability. Various lysosomal enzymes other than sialidase were hardly affected by the transformation. Sialic acid transfer to N-linked glycoproteins was slightly elevated in the transformants, but not in parallel with their metastatic potential.